The xenon lamps used as a light source in weathermeters, fademeters and lightfastness testing devices in general suffer degradation with use and gradually diminish in the quantity of light they produce. This is unfortunate, because maintaining the quantity of light constant at all times for the purpose of conducting these tests is desirable.
Japanese Utility Model Applications Nos. 47-021182, 47-021183 and 50-058384, filed by the Applicant of the present application, disclose methods of recovering energy at a constant rate by incorporating a temperature difference radiometer and other light-receiving elements into the testing apparatus in order to adjust the voltage applied to the xenon lamp by means of a control circuit based on the output of said light-receiving element.
Such methods, however, do not consider compensating for the temperature of the light-receiving end; and therefore, the surrounding temperature of the light-receiving element rises due to the element's absorption of light. Furthermore, when optical fibers are used, the degradation in the weatherability of the coating material also affects the device, despite the fact that optical fibers are designed so as not to produce light losses by utilizing a different reflective index between the coating material and the fiber.
A temperature change of 10.degree. C. caused by the aforesaid effects corresponds to a change in light quantity of about 10%, and results in a halving of the light-introducing efficiency within 1 to 2 years.